A Closer Look at the City in the Forest

Trees provide important public health, social, environmental, and economic benefits to humans and the environment. Clean air and water, reduction of greenhouse gases through carbon sequestration, energy conservation, improved wildlife habitat, and better mental and physical health are just a few examples of the benefits provided by trees.

Many cities conduct urban tree canopy assessments (UTC) to better understand these important natural resources and their benefits. Tree canopy, generally described as tree leaves, branches and stems that cover the ground when viewed from above, is most often derived from analysis of aerial photography or satellite imagery.

Urban tree canopy assessments, which measure canopy quantity not quality and do not provide a count of individual trees, can be used to inform decision-making, policy and sustainability efforts.

The City of Atlanta Department of Planning completed their first ever city-wide analysis of tree canopy utilizing 2008 satellite imagery. The baseline analysis revealed that Atlanta’s overall tree canopy was  number one  in coverage for a major metropolitan city at 47.9%. The baseline study also found that canopy coverage in the city varies tremendously, from less than 10% downtown and along transportation corridors to over 90% in nature preserves and along some stream corridors.

In 2018, the City of Atlanta conducted its 3rd canopy assessment and estimated tree canopy coverage to be 46.5% (40,609 acres) of the city's total area, excluding the airport. The 2018 canopy trends and results were similar to findings from the previous two assessments -- urban tree canopy coverage is driven by land use and varies significantly across the city, from less than 10% downtown to over 90% in nature preserves. As seen in the map to the left, canopy coverage is extremely low at the city center and slowly gains coverage as one moves outward from downtown.

The majority of canopy (75.5%) is found on single-family residential land, followed by multi-family residential (8.0%) and industrial (5.8%). Several neighborhoods have more than 70% tree cover, with Boulder Park having the highest (82.9%). Many of the downtown neighborhoods have less than 15% canopy coverage, with the Oakland neighborhood having the lowest percent tree canopy (3.2%). Check out some of the 2018 canopy maps and land cover graphs below.

The City of Atlanta 2018 percent urban tree canopy coverage for 2018 was 46.5%, a slight decrease from 2014 and almost 1.5% less than 2008, which equated to an estimated .43 acres of canopy lost per day between 2008 - 2018. It should be noted that the city annexed almost 3,000 acres of land between 2008 – 2018 (see 1st map below). Consequently, the changes in canopy area and percent tree cover between 2008-2018 period are a bit misrepresented at the city-scale. If we calculate 2018 canopy using the 2008 city limits, the 2018 percent canopy cover drops to 46%, indicating that land annexed after 2008 is majority forested.

Regardless, canopy change varies greatly across the city, with a majority of loss coming from the northern parts of the city. Check out some of the maps and graphs below for a closer look at canopy change across the city between 2008 - 2018.

The great variation in canopy change warranted a more detailed examination of the causes, which included over 100 in-person site visits and over 500 remote observation across the City. This effort was intended to validate the results and "qualify" the observed changes into tree growth and loss categories. The process began with identification of 6-acre grid cells from a city-wide grid that showed change of + or - 1 acre in canopy (1st map below). From these results, multiple areas were selected for site visits. The maps below highlight these sites and their growth and loss categorizations.

As seen in the maps above, the vast majority of areas showing loss of canopy are located in the northern parts of the city. Single-family redevelopment was the most common cause of loss, followed by construction of new developments (single and multi-family housing, industrial, mixed use, institutional and commercial). While all of these sites have replanted trees as required, the new trees are generally not the same quality or type as the trees lost. Check out pictures below for examples of canopy loss.

Canopy gain in the City of Atlanta is more complicated to track and quantify than canopy loss. Canopy gain was observed across the city, as expected, as all trees experience growth, with younger trees growing faster than older, mature trees. However, most of the significant canopy growth observed between 2008-201, which occurred almost exclusively south of I-20, was actually regrowth of forest on lands cleared for development but left undeveloped or partially developed (Pipe Farms) and, regrowth on former Atlanta Housing Authority sites that have been demolished yet remain undeveloped. Significant canopy growth was also observed in younger street trees and neighborhood trees planted in single-family neighborhoods and multi-family developments constructed between 2000 - 2008. There was also some significant growth observed in a few City of Atlanta parks. All of these areas are good representations of how much very young trees can grow in the City of Atlanta in 20 years of less. Check out the pictures below for examples of canopy gain between 2008 - 2018.

As described above, many of the areas showing significant canopy gain were sites that had been cleared for development sometime between 2000 and 2008 yet remained undeveloped or partially developed through 2018, during which time regrowth of trees occurred, and often times covering all of the previously cleared lands except those that were paved. Interestingly enough, some of these sites showing no canopy in 2008 and gain in 2014 were again cleared in 2018 once development recommenced. While it is a good thing that theses areas were eventually developed instead of clearing another canopied area, it should be noted that this type of growth is "temporary" and possibly skews the overall canopy values for 2018 and for future assessments. Check out the pictures below for examples of these "loss gain loss" sites.

If you are interested in exploring these sites and viewing the observed changes on your own, check out the  2018 Site Visit Explorer  and  2008 - 2018 Urban Tree Canopy Change Swipe Application  below.

Most of the city's trees are on private lands. Unfortunately, canopy loss is outpacing canopy gain and the gap between quality canopy and overall loss is widening. With development steadily increasing since 2012, the likelihood of increased tree loss is ever present, especially since some zoning categories permit complete or almost complete removal of trees

To maintain the existing canopy, the options are simple: plant more trees, mitigate loss and permanently protect existing forests, especially large, contiguous tracts.

Mitigate Canopy Loss

The best way to preserve the existing canopy is to mitigate loss. This can be accomplished by

• permanently protecting existing forests
• modifying the zoning code to limit max lot coverage or tree removal
• permanently protect trees of a certain calipher
• educate the public on the importance of preserving urban tree canopy

In addition to loss mitigation, tree planting can help maintain existing canopy. But offsetting canopy loss through tree planting can be difficult. It also takes significant time to reestablish mature canopy.

Several years ago, the City of Atlanta set a goal of 50% canopy coverage. With a 2018 canopy coverage of 46.5% (40,609) acres, 3,000 acres of canopy are needed to reach that goal. There are approximately 1,300 acres of non-tree vegetation ( NTV) on existing park land, 300 acres on Atlanta Public Schools (APS) land, and 800 acres of "other" public lands (GDOT, MARTA, Fulton County). If 10% of this land was use for tree plantings, that equates to 240 acres of new canopy. There are, however, roughly 15,000 acres of NTV on private land. Incentives to businesses and citizens to protect existing trees and plant new ones would be beneficial and would help offset future canopy loss. Know a good place for a tree or two? Contact  Trees Atlanta  for some help! Need help finding a place for planting trees? Check out the Potential Planting Index below.

Using results from the 2018 City of Atlanta Urban Tree Canopy Assessment, the project team created a Potential Planting Index (PPI) for the city. The PPI was calculated across a city-wide grid comprised of 6-acre cells containing aggregated 2018 urban tree canopy (UTC), non-tree vegetation (NTV) and non-vegetation (NV) land cover statistics.

Potential Planting Index =%NTV - % Canopy

PPI grid cell values range from -1 (very low potential, large amounts of existing forest) to + 1 (high potential, no forest and plenty of non-tree vegetation like grass, bushes, shrubs, etc. ).

While not a substitute for other methods of tree planting area identification, the PPI is a planting area identification supplement that can be used to quickly assesses areas across the city for their tree planting potential, and all based on data obtained from the 2018 canopy assessment.

f you'd like to learn more about the PPI and explore some custom PPI tools for yourself, please visit our Potential Planting Areas Story Map below.

If you'd like to explore the 2018 Urban Tree Canopy Assessment and Change Analysis data on your own, please visit our customized ESRI mapping application linked below. This interactive mapping application allows users to view maps and project data, query areas for 2018 canopy and 2008-2018 canopy change statistics, view 2018 canopy charts, and print and share your own custom canopy maps.

Using conversion factors developed by the USDA Forest Service and available through their online tool,  I-Tree Canopy , the project team calculated estimates of Atlanta's carbon, air pollution and hydrological ecosystem service benefits provided by the city's urban tree canopy. The results of these 2018 ecosystem service benefit estimates ae presented in a series of ESRI data dashboards accessible below. View ecosystem benefit estimates by neighborhood, NPU, watershed, small watershed, and city council district by clicking the corresponding link below.

Thank you for your interest in Atlanta's urban tree canopy. For more information, please contact Kathy Evans (kaevans@atlantaga.gov) in the  City of Atlanta Arborist Division  or Tony Giarrusso (tonyg@gatech.edu) with  the Center for Spatial Planning Analytics and Visualization at Georgia Tech .